The present invention relates to printing onto PVC (i.e. polyvinyl chloride) materials, particularly flexible (i.e. plasticized) PVC film and sheeting.
Successful processing of PVC plastic film and sheeting using metal equipment such as extruders, roll mills, or calendars requires lubrication, i.e. the maintenance of a thin (ideally monomolecular) film of material between the metal and the plastic to reduce friction and prevent sticking of the plastic to the metal. Consequently, there is usually some lubricant-derived foreign material present on the surface of the PVC film or sheet as a separate phase into which minor components of the plastic (components and impurities in plasticizers, stabilizers, etc) may migrate. At the same time, successful printing requires first wetting of the plastic surface by the ink and then bonding of the ink's binder to the plastic to ensure that the print does not rub off or wash off. Any foreign material present on the surface of the PVC may constitute a barrier to the required wetting and bonding of the printing ink. Successful lubrication and printing therefore require careful balancing of conflicting influences.
A further complication is the likelihood of a time lag between the initial shaping of the plastic into e.g. film or sheet form, and printing, as quantities of unprinted PVC material may be shipped to off-site printing plants and warehoused for some time, possibly under warm and humid conditions, before being printed. In fact it has been observed that some PVC materials can be printed successfully when fresh but diminish in printability upon storage.